Medical Device Development
ResMed is a billion dollar company specializing in development and manufacture of products for screening, treatment, and long-term management of sleep-disordered breathing and other respiratory ailments. ResMed's products provide therapeutic relief for a variety of breathing disorders; the products are based on complex algorithms that rely on patient interaction for effective treatment. In product testing, such interactions present problems because traditional breathing simulators provide limited physiological realism and zero response to therapy. The alternative — clinical trials on real patients — is slow, expensive, and has low repeatability.
These limitations prompted ResMed to develop breathing simulators based on more sophisticated computer models of physiology, using VisSim. VisSim has demonstrated benefits to ResMed for all stages of product development, from initial algorithm testing and product testing with virtual patients, to clinical education.
VisSim is used to create both virtual and physical simulators. ResMed's extensive library of VisSim-based virtual patients covers a wide range of target pathologies, including obstructive sleep apnea, neuromuscular disease, and congestive heart failure (Cheyne-Stokes respiration).
VisSim as Virtual Patient
VisSim was first used by ResMed in the development of a "bench patient" — a simulated patient with periodic breathing during sleep — to respond to its adaptive servo-ventilation product. In this case, a personal computer running the VisSim simulation in real time was linked to a customized test lung, which in turn was connected to the ResMed therapy device (see picture at right). In this hardware-in-the-loop simulation, the PC closed the loop around the hardware test lung such that the bench patient's response to therapy was very similar to that of a real patient with the target pathology.
This therapy response was achieved by detailed high-fidelity modeling of the underlying human physiology coupled with VisSim's fast execution capability, which allowed computational response in real time. The VisSim-based bench patient greatly accelerated equivalence testing of ResMed's second-generation therapy device. The result was shorter clinical trials and reduced time to market.
Speeding Up the Simulation with VisSim/C-Code
ResMed continued to expand the functionality of its simulated patients by incorporating a collapsible upper-airway and pulsatile heart model. The downside of this added complexity was that VisSim's normal interpreted simulation speed could no longer run in real time. To restore real-time (or faster) capability, ResMed used the VisSim/C-Code add-on module to automatically generate ANSI C code from major subsystems of the VisSim diagram. Each subsystem was replaced by a VisSim-generated DLL that ran up to five times faster than its block diagram counterpart.
Using VisSim/C-Code on an RT-Linux Embedded Target
By purchasing the source code to the VisSim C-Code support library, ResMed was able to bring its own models to any embedded platform. Using RTAI Linux and the VisSim generated code, ResMed was able to perform its product testing on high bandwidth patient models.
Complete Interactive Virtual Patient and Therapy Device
The VisSim physiology models are also used as a clinical training simulator. Using VisSim's built-in blocks for animation, plotting, and user interface, VisSim models of the device's user interface panel, therapy device, and air delivery circuit were created and added to models of the therapy device, thus creating an entire virtual system: a virtual therapy device treating a virtual patient (see diagrams at left). Customers and sales personnel interact with the simulated device in the same way as in a clinical setting. By adjusting the therapy, the user can see real-time patient outcomes, including respiration, blood gas tensions, and the patient's respiratory effort. The free VisSim Viewer makes the simulation readily available for distribution via CD or internet to ResMed's global clients.
ResMed sees its continuing use of VisSim as beneficial in simultaneously maximizing quality, minimizing development time, and enhancing customer experience through use of realistic patient models.